Some of the most interesting kitchen chemistry can be observed when baking. Taking four basic ingredients - flour, fat, sugar and eggs - and subtly altering their cooking chemistry can transform them into airy cakes, chewy cookies or flaky pastries.
Leavening, or raising, agents introduce bubbles of air. As the air bubbles are heated, the gas that they contain expands, causing cakes, breads and soufflés to rise. These air bubbles can be made in one of two ways. Chemical raising agents, like baking powder and bicarbonate of soda, react with water to form carbon dioxide gas. This reaction occurs very rapidly and the quantity of raising agent must be carefully adjusted - too much and the bubbles will become large and burst, too little and the density of the cake mixture will prevent any bubble formation at all.
For a slower rise with added flavour, baker's yeast (Saccharomyces cerevisiae) is often used. Yeast is a single-celled organism of the fungi family. At first, the yeast respires aerobically - using oxygen - creating bubbles of carbon dioxide. When the oxygen runs out, the yeast begins to make ethanol by fermentation, much like in brewing beer, but any alcohol formed in the bread dough evaporates in the oven.
Making bubbles is one thing, but getting them to remain intact requires more clever chemistry. Bread is most often made from wheat flour, which contains starch granules surrounded by two important proteins: glutenin and gliadin. When mixed with water and kneaded, the glutenin cross-links to form networks with gliadin, making a new stretchy protein: gluten. Gluten is a 'super-protein', or protein complex, which behaves much like elastic, forming stretchy bridges that hold the starch molecules together. The key to light, fluffy bread lies in creating lots of tiny elastic bubbles; the more the dough is kneaded and stretched, the stronger the gluten network becomes. Eggs act in a similar way to the gluten in flour, providing a protein-binding agent that supports air bubbles and holds cakes together.
Unlike bread, pastries need to be 'short' and crumbly, so bakers try to minimise gluten production, which would lead to a rubbery texture. This is done by first rubbing butter into the flour, coating the starch molecules with a layer of fat, which helps prevent glutenin and gliadin from coming into contact with water. The texture of baked goods can also be altered using sugar. When sugar is beaten with butter, the sharp edges of the sugar crystals allow tiny air bubbles to form - turning the mixture a pale, creamy yellow colour. These bubbles expand in the same way as the ones created by raising agents, contributing to the light texture of cakes. For the denser consistency of cookies, melted fats and oils are often used because the tendency for bubbles to form next to the sugar crystals is reduced.
Sugar also draws in moisture from the air, which can have a significant effect on the water content of baked goods. Brown sugar attracts more water than white, and finely ground sugars attract more water than the granulated variety. Experimenting with the type of sugar used in a recipe will alter the final moisture content, and therefore the texture.
Chemistry isn't just limited to baking though. Chemical reactions define the taste of meat - which is around 70 per cent water, with the remainder being mostly protein and fat. Depending on the cut, meat contains a variable amount of collagen - a fibrous protein in the skin, tendons and connective tissue. The higher the collagen content, the tougher the meat is. More expensive cuts and meat from younger animals contain little collagen and can be cooked rapidly. The muscle protein myosin denatures (breaks downs) at low temperatures - i.e. 50 degrees Celsius (120 degrees Fahrenheit) - and begins to form cross-links, lending some support to the structure of meat. At this stage, water molecules between the proteins start to leak out, but the meat remains juicy and tender. At 60 degrees Celsius (140 degrees Fahrenheit) the red pigment in muscle - myoglobin - denatures to form a hemichrome that gives cooked red meat its brown-grey colour. Further heating causes the collagen to shrink and contract, forcing water out and turning the meat from juicy and tender to chewy and dry. If the temperature is raised still further - to, say, 70 degrees Celsius (160 degrees Fahrenheit) - the meat continues to toughen, but the collagen itself dissolves to form gelatine. Although the fibres of meat are more brittle, the gelatine acts as a lubricant, giving slow-cooked meat its soft, 'melt-in-the-mouth' texture.
Heat is not the only way to break down collagen, though, and meat can be physically or chemically tenderised. Marinades use common culinary chemicals to interfere with the bonds between collagen strands - these range from acids like lemon juice to enzymes like bromelain (found in pineapple).
(from "How it Works?")